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When changing the transmission fluid in a vehicle with 150,000 miles, it's important to consider both the condition of the fluid and the recommendations of the vehicle manufacturer. As vehicles accumulate mileage, the transmission fluid can become contaminated or degraded, which negatively impacts transmission performance and longevity.

In this situation, assessing the existing transmission fluid color and odor can provide clues about its condition. Fresh transmission fluid is typically red and has a slightly sweet smell, while old fluid can appear dark brown or even black, and may have a burnt smell. This assessment is crucial before deciding to change the fluid.

Additionally, consulting the manufacturer's service guidelines is essential. Manufacturers often have specific recommendations regarding fluid change intervals based on the vehicle’s mileage and usage patterns. Following these guidelines helps ensure the vehicle operates efficiently and maintains the integrity of the transmission.

Both of these factors—evaluating the condition of the fluid and adhering to the manufacturer's recommendations—are fundamental when addressing transmission fluid maintenance in high-mileage vehicles.